Szabolcs Horvát wrote:
> Why is NonCommutativeMultiply Protected?
>
> Is it safe to redefine NonCommutativeMultiply? Does it have any special
> built-in properties (apart from being Flat and OneIdentity)? Is it used
> by any of the built-in functions?
>
> NonCommutativeMultiply certainly does not evaluate to anything, but the
> same can be said about Rule[], which is not at all safe to redefine
> because: 1. It has some special properties (it affects scoping, it does
> not like Unevaluated[], etc.) 2. It is used together with functions
> such as Replace, and (most probably) built-in functions use it internally.
>
> So is it safe to Unprotect[NonCommutativeMultiply] and attach
> definitions to it? (I know that in certain situations one can use
> UpValues, but I would like to know the answer to this question.)
>
Yes, NonCommutativeMultiply is meant for user use, so go ahead and
Unprotect it. It is, however, extremely ugly, and might get confused
with the Fortran exponentiation operator! I prefer to use CircleTimes,
or another built-in operator and give it the same attributes as
NonCommutativeMultiply.
David Bailey
http://www.dbaileyconsultancy.co.uk